Download P. Ventura

Motivation
Stars belonging to most of the GCs ARE NOT
chemically homogeneous: observations
evidentiate clear star to star variations among
the surface abundances.
Self-pollution scenario: these anomalies can be
explained via the pollution provided by the
ejecta of an early generation of intermediate
mass stars evolving on time scales of the order
of 100-200 Myr.
• The Na-O
anticorrelations
is found in the
totality of GCs
studied.
• Field stars are
much more
homogeneous
Sneden et al (2004)
Magnesium – Aluminum anticorrelation
Sneden et al (2004)
Sodium – Aluminum correlation
Sneden et al (2004)
Chemical anomalies
The stars chemically “anomalous” show surface
abundances that, when compared to their
“standard” counterparts, are:
a) Sodium and Aluminum rich.
b) Oxygen and Fluorine poor.
c) Moderately Magnesium poor.
d) The C+N+O sum is approximately constant.
Can AGB ejecta keep account of points
(a) – (d) ??
The AGB models are extremely
sensitive to all the uncertainties
affecting stellar Astrophysics
* Mass loss
* Extra-mixing from the bottom of the
surface convective zone
* Nuclear cross-sections
* CONVECTION (FST: Full Spectrum of
Turbulence vs MLT: Mixing Length Theory)
The role of convection
A convective model of higher efficiency favours
Oxygen, Magnesium and Sodium destruction.
In the FST models Fluorine is heavily destroyed
during the early TPs phase. Aluminum is produced.
FST Models
Good
* Oxygen and Fluorine
are heavily depleted
* Aluminum is produced
consistently with the
observations
* Magnesium is globally
depleted
* The C+N+O is
constant within a
factor of 2 in all
models
Bad
* Sodium is not
produced in the most
massive models!
* In models M>4.5Msun
the magnesium isotopic
ratios are too large
The impact of mass loss
Increasing the
mass loss rate ...
a) decreases the
total AGB lifetime.
b) reduces the
total number of
TPs (TDU!)
c) The maximum
luminosity (hence
Tbce) is lower
Those elements
destroyed via HBB
(Na23,O16,Mg24,
F19) can survive
more for higher
Mdot.
Higher Mdot
lowers the
abundances of
those elements
dredged-up in the
after TP phase
(C12, Ne22)
Fenner et al. 2004 (MLT)
Modelli FST + mass loss
enhanced
Ventura & D’Antona
2005 (A&A 431,
279) (FST)
Ne22+p->Na23
II dredge-up
Na23+p
III dredge-up(Ne22)
The surface abundance of Sodium is generally a
result of various processes typical of the AGB
evolution.
Extra-mixing
Including extra-mixing adds a free parameter
but..ζ(MS)=0.02-0.03 !!
Fenner et al. 2004 (MLT)
Modelli FST + mass loss
enhanced
Ventura & D’Antona
2005 (A&A 431,279)
(FST)
* Extra-mixing from the bottom of the convective
envelope in the after-TP phase favours a deeper
penetration of the surface convection within
regions previously touched by He-burning.
* The abundances of C12, O16 and Ne22 are
enhanced, as well as the total C+N+O sum.
* Sodium production is also favoured via a later
proton capture by Ne22 nuclei.
* A large extra-mixing favours a great increase of
the total C+N+O and renders difficult a net O16
destruction.
* The abundances of Fluorine and Aluminum are
almost independent of the assumptions regarding
extra-mixing.
Convection
Extra-mixing
5Msun FST std
Mass loss
Conclusions
* The FST models with a modest
amount of extra-mixing can account
for almost all the chemical anomalies
observed in GCs stars.
* The Magnesium isotopic ratios problem
is still open.
* The predictive power of the AGB
models, and particularly the chemical
content of their ejecta, is still
extremely low.
Stellar generations in the Na-O plane
Evolved stars?
Second
generation
formed by
AGB winds
First
generation:
Original
abundances
Interesting results
1) CARBON STARS composition for massive
AGBs of very low Z!! (But... The lowest Z
clusters –M15, M92- do not show incredible
abundance anomalies: something PREVENTS
an efficient second generation star formation
(or the models are wrong)
2) the third dredge up raises the total CNO
abundance in the envelope: but remember
that –observationally- C+N+O is roughly
constant in spite of the huge variations: this
means that the TDU must not be very
efficient (e.g. no serious overshooting)
Sintesi di Sodio in stelle AGB
If ‘selfpollution’ is simply successive
stellar generations formed from ejecta
The patterns of inhomogeneities have a “simple”
key of interpretation:
The mass evolving in the first epoch is a simple
function of the time. The chemistry of the
ejecta, and thus of the formed stars, is simply
the chemistry of the ejected AGB envelope,
which depends on the initial mass…
The dispersion in GC stars composition IS NOT
random, it must resemble the distribution of
the yields as function of the total mass…
Modelli di 5
masse solari
con Z=0.001
Extra - mixing
Standard
L’extra mixing adottato è solo 1/20 di
quello usato normalmente in sequenza
L’anticorrelazione
O-Na è presente
nelle stelle di
quasi tutti gli
ammassi globulari
esaminati
La Convezione porta i prodotti
della combustione nucleare in
superficie
Atmosfera
Inviluppo convettivo (H
-rich)
(H-rich)
Shell CNO
T>4 107K: reazioni nucleari
He intershell
Nucleo C
-O
C-O
Correlazione Alluminio - Sodio
L’anticorrelazione
Mg-Al in NGC6752.
Pannello in alto: stelle TO
Pannello in basso: Subgiganti
Le stelle ricche di Sodio
sono ricche in Alluminio e
con poco Magnesio
Nucleosintesi CNO avanzata
28Si
25Al 26Al27Al
23Mg 24Mg25Mg26Mg
21Na 22Na 23Na
20Ne 21Ne 22Ne
17F 18F 19F
16O 17O 18O
Fin dai primissimi pulsi l’evoluzione in luminosità
dei modelli è sensibilmente diversa, nonostante i
valori di picco della produzione energetica
nucleare CNO siano molto simili … Perché?
Picco nucleare CNO
Base inviluppo
Picco secondario
NGC 6397 e NGC 6752 - Stelle selezionate:
14 stelle TO e 12 subgiganti (sotto il clump!)
L’anticorrelazione
O-Na in NGC6752.
Punti pieni: stelle TO
Punti vuoti: subgiganti
Evoluzione di un modello di 5 masse solari
durante la fase dei pulsi termici
Evoluzione chimica superficiale di un modello di
5 masse solari durante la fase dei pulsi termici
Modelli AGB sviluppati da altri gruppi mostrano:
a) L’Ossigeno non viene toccato dalla nucleosintesi
alla base dell’inviluppo esterno
b) Il Sodio viene prodotto in quantità di gran lunga
eccedenti quelle richieste
c) La somma C+N+O aumenta vertiginosamente
“The notion that massive AGBs are the origin of
the O-Na anticorrelation in GCs stars is not
consistent with the model predictions of this
study” (Herwig 2004)
“The current theoretical yields do not favour
the AGB pollution scenario as the mechanism
responsible for star-to-star abundance
variations in GCs” (Fenner et al. 2004)